The present invention relates to improved systems for monitoring the charge condition of an electric storage battery such as a lead acid battery. The invention is particularly useful for conditions of operation where varying loads are intermittently applied to the battery, and where the battery may be intermittently subjected to brief intervals of charge current.
Many different approaches have been made to the problem of monitoring and indicating the discharge condition of electric storage batteries, especially in applications where the battery is first charged and then is used in the discharge mode for a considerable period of time, such as in a mobile vehicle, before it is again returned to the charger to be recharged.
One valuable approach to this problem is to use an ampere-hour meter. Very accurate results have been obtained with such meters. However, accuracy is sometimes limited by the fact that the number of ampere hours obtainable from a battery depends very much upon the rate of discharge, fewer ampere hours being available when the battery is rapidly discharged. Other factors which limit a system of pure current measurement are that the amount of energy available from the battery varies with the temperaure of the battery, and with the age of the battery. Furthermore, the ampere-hour meter approach requires insertion in the circuit of some means for measuring current, such as a current measuring shunt. That is not nearly so convenient as simply measuring the battery terminal voltage.
Various battery terminal voltage measurement systems have been employed for the purpose of monitoring battery discharge, with varying degrees of success. The present invention is in this category.
One voltage measurement technique for monitoring battery discharge consists of simply measuring the open circuit battery voltage. The stabilized open circuit battery voltage can be an accurate indication of the discharge state of the battery. However, it may take hours for the open circuit battery voltage to stabilize after each discharge interval, so that the stabilized reading is not available very much of the time. Furthermore, no open circuit battery voltage is available during a loaded condition of the battery.
Another approach is simply to measure and read the instantaneous battery terminal voltage on a continuing basis. However, this requires interpretation, since the terminal voltage varies during operation of the apparatus powered by the battery, having a depressed value during loading, dependent upon the magnitude of the load.
Various unidirectional systems have been devised for detecting and registering downward excursions in battery voltage under load as an indication of the discharge condition of the battery. Some of these have operated on the basis of recognition of a sustained under-voltage condition before any registration is made, and others are really under-voltage analyzers which recognize and register under-voltage conditions in a setting recognizing previously registered under-voltage conditions. Very accurate results have been obtained with such systems, especially when applied to apparatus having known patterns of loading, even where the loading is quite discontinuous, such as in the operation of industrial forklift trucks. Examples of successful systems of this sort are disclosed and claimed in a prior U.S. Pat. No. 4,193,026 issued to Eugene P. Finger and Eugene A. Sands on Mar. 11, 1980 and assigned to the same assignee as the present application.
Another example of a monitoring system in the above category is that disclosed in U.S. Pat. No. 4,560,937 issued on Dec. 24, 1985 to Eugene P. Finger for a Battery State of Charge Metering Method and Apparatus and assigned to the same assignee as the present application.
The above-mentioned prior patents basically involve unidirectional integrations in response to battery terminal voltage reduction events associated with discharge events on the battery. While the systems in each of these two patents include circuit features for recognizing when the battery has been recharged, and for resetting the metering system to reflect that condition, there is no means for measuring the degree of recharge during partial recharging intervals. Thus, the systems of those patents assume that a full charge is accomplished, if prescribed tests are passed. They contemplate that the discharged battery is to be removed and replaced each time with a recharged battery. There is no recognition of a partial recharge, or any provision for bidirectional integration.
Another prior patent which represents a refinement in battery state-of-charge indicators is U.S. Pat. No. 4,388,618 issued June 14, 1983 to Eugene P. Finger for a Battery State-of-Charge Indicator Operating on Bidirectional Integrations of Terminal Voltage and assigned to the same assignee as the present application.
U.S. Pat. No. 4,388,618 does include the feature of bidirectional integration of terminal voltage for the purpose of deriving an accurate measurement of battery state of charge. However, the system of that patent again basically contemplates a battery application where the battery is expected to be removed from the battery utilization apparatus, such as a forklift truck, and then replaced with a recharged battery. The bidirectional integrations of the system of U.S. Pat. No. 4,388,618 including upward voltage responsive integrations, are intended to provide for recognition of the beneficial effect of "rest" periods in the operation of the battery in which the battery terminal voltage actually increases in a voltage recovery characteristic after loading. U.S. Pat. No. 4,388,618 again includes a special reset arrangemennt for quickly resetting the integrator to recognize that the battery has been recharged after the battery is removed, recharged, and replaced.
While U.S. Pat. No. 4,388,618 system operates bidirectionally, and recognizes upward voltage excursions, it is specifically designed only to recognize and respond to upward voltage excursions associated with rest intervals between discharge events, and to respond accurately to such rest conditions. It is not designed to respond to brief, or prolonged, recharge events, and therefore will not respond accurately to such recharge events. For instance, since the system is designed to respond to mere quiescent voltage recoveries when the load is removed, it is too sensitive, and corrects too much in response to brief charging events such as those arising from regenerative braking.
Accordingly, it is an object of the invention to provide an improved battery state-of-charge monitoring circuit which responds accurately to brief charge condition events as well as to brief discharge condition events.
Still other atempts have been made at producing a battery state-of-charge circuit which responds to both charging conditions and discharging conditions. For instance, U.S. Pat. No. 4,258,306 issued Mar. 24, 1981 to Robert F. Bourke and David K. Johansen for a State of Battery Charge Indicator Circuit and assigned to Gould Inc. discloses such a system. However, the system disclosed in that patent apparently requires that the operator insert an input signal to impart the information that the system is in charge mode. In other words, the shift between charge mode and discharge mode is not automatically detected. Furthermore, the system of that patent requires the presence of a current detection signal during discharge mode, but not during charge mode. This is a disadvantage, since it means that current must be detected, and arrangements for detecting current make installation of the state of-charge-circuit more difficult.
Accordingly, it is another object of the invention to provide an improved battery state-of-charge monitor circuit which automatically determines when the system shifts between charge mode and discharge mode, and which does not require any measurement of current beyond those indications of current which are implied from detection of battery terminal voltage.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.